Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse

Abstract

The (ultra)high-pressure Western Gneiss Region of the Norwegian Caledonides represents an archetypical orogenic infrastructure of a continent–continent collision zone. To test established exhumation models, we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas. Migmatite U–Pb zircon samples date melt crystallization at c. 405 Ma in the Øygarden Complex, expanding the spatial extent of Devonian migmatization. Micas from shear zones in the Øygarden and Gulen domes yield 40Ar/39Ar ages mostly between 405 and 398 Ma, recording the exhumation of metamorphic core complexes. On a larger scale, the youngest ages of various geochronometers in different segments of the Western Gneiss Region show abrupt breaks (10–30 myr) across low-angle detachments and sinistral transfer zones, which also correspond to metamorphic and structural discontinuities. We explain the segmentation of the orogenic infrastructure by partitioned post-orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice (strong cratonic foreland and orogenic wedge v. soft infrastructure). Differential crustal stretching dragged out deep levels of the orogenic crust below low-angle detachments and became progressively dominated by sinistral transfer zones. Collapse obliterated the syn-collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments.